The minimum reservoir capacity necessary to avoid air-dilution. An experimental model of spontaneous breathing through a T-piece.

The fresh gas flow rate necessary to prevent rebreathing in T-piece anaesthesia is well established at twice total ventilation. The minimum reservoir volume to avoid air dilution of anaesthetic gases in the alveolus at this flow rate is, however, undecided. A lung model was constructed to represent spontaneous respiration, and the value of minimum reservoir volume as a proportion of tidal volume determined experimentally. The values so obtained were in close accord with those calculated theoretically. Extension of the theoretical analysis leads to a recommendation that a reservoir volume one-third that of tidal volume, as originally suggested by Ayre in 1956, will prevent air-dilution.